A cell surface glycoprotein, fibronectin, is involved in cell adhesion in human skin fibroblasts. Fibronectin is not involved in the initiation of cell proliferation, but is involved in the events during and following cell proliferation. During cellular aging, changes at the cell surface and in the fibronectin have been reported. This proposal will evaluate the amount of intracellular, cell surface-associated and secreted fibronectin in human skin fibroblasts before and after the cells lose their proliferative capacity. In order to determine the events which regulate the fibronectin in human skin fibroblasts during in vitro aging, the kinetics of synthesis, transit to the cell surface, turnover and secretion will be determined. An inverse relationship exists between the amount of cell surface protease activity and the expression of cell surface fibronectin. During mitosis the maximum amount of cell surface protease activity is present in proliferating human skin fibroblasts, while a decrease in cell surface fibronectin has been measured. In non-proliferating fibroblasts, increased levels of fibronectin have also been observed while decreases in cell surface protease activity have been observed. The levels of cellular protease activity plus secreted protease activity will be measured during the various stages of proliferative activity in aging human skin fibroblasts. The amounts of cellular proteases in aging human skin fibroblasts will increase our understanding of the degradation process. This proposal will investigate the regulation and structure of one of the major glycoproteins of human skin fibroblasts during cellular aging. The information provided in the proposal may be essential toward our understanding of the mechanism resulting in the loss of proliferative capacity of normal human fibroblasts-like cells in vitro and in vivo.